Attention has been paid to optical modulators in which a PIN (P-intrinsic-N) junction or the like is formed in a silicon optical waveguide and the carrier density is changed to modulate the refractive index of the region. This is because the optical modulators have various advantages such as small size, low voltage driving, low cost, and easiness to be integrated with other optical elements and electronic circuits as compared with lithium-niobate optical modulators which have been used as optical modulators. It is known that this type of optical modulator can provide a high efficiency during forward biasing on the PIN junction but the efficiency decreases at 20 dB/decade in the frequency range higher than the cut-off frequency which is determined by the CR time constant. Non Patent Literature (S. Akiyama, et al., “12.5-Gb/s operation with 0.29-Vcm VπL using silicon Mach-Zehnder modulator based-on forward-biased pin diode,” OPTICS EXPRESS, Vol. 20, No. 3, pp. 2911-2923, 2012) discloses a method for pre-emphasizing an input electrical signal with an FIR (Finite Impulse Response) filter beforehand in order to compensate for such frequency characteristics.
On the other hand, Japanese Patent Application Laid-open Publication No. 2012-063768 discloses an optical modulator capable of generating an optical de-emphasis signal.
Related techniques are described in Japanese Patent Application Laid-open Publication No. 2002-258228, Japanese Patent Application Laid-open Publication No. 2004-085602, and International Publication Number WO2011/114753.